The effects of hypoxia and high energy phosphate depletion on contraction and relaxation in chick embryo ventricular cells cultures as confluent, spontaneously contracting monolayers will be studied. The influence of depletion of high energy phosphates by exposure to cyanide and by exposure to low PO2 adequate to casuse negative inotropic effects in these cells on calcium influx occurring via sodium-calcium exchange and via the slow channel will be determined. In addition, the influence of high energy phosphate depletion on calcium influx occurring in the cells will be measured. Changes in calcium content will be correlated with alterations in relaxation in these cells to determine if calcium overload is involved in the abnormal relaxation process seen during hypoxia. In addition, the relative importance of high energy phosphates derived from oxidative phosphorylation versus high energy phosphates derived from glycolysis on monovalent cation active transport will be determined, by measuring uptake of K42 and Rb86. Our overall goal is to determine the mechanisms of the negative inotropic effects of hypoxia and to investigate the causes of abnormal relaxation during hypoxia.